Advanced Timber Knowledge

Why Does Timber Move?

Realising that all timber will move regardless of the species and environment can be an unpleasant surprise.  But as the expression goes, forewarned is forearmed.  Understanding why timber moves and how is crucial when working with and specifying timber. 

What is meant by movement? 

Timber movement, or warping, is where the timber is curved in some plane because of uneven shrinkage or expansion.  It usually affects planks and smaller sections of timber, although larger profiles can be affected as well.  Movement is an umbrella term for a range of specific movements.  The below list describes these in more detail: 

  • Bow: A curve along the length of the edge of the wood. 
  • Crook: A curve along the length of the face of the wood. 
  • Kink: A localised warp, often caused by a knot 
  • Cup: Curved in a U-Profile, where the sides of the face are higher/lower than the centre of the wood. 
  • Twist: A move in the length of the timber, resulting in the two ends not lying flush on an even surface. 

Why does Timber move? 

Timber is a ‘hygroscopic’ material, which means that is absorbs moisture from or releases moisture into the atmosphere until it achieves a balance with its surroundings. 

At any given moment, there is a dynamic exchange of moisture occurring between the timber and the air of the environment.  The moisture is transferred in the form of water vapour by the process of diffusion.  This diffusion is driven by differences in the moisture content of the air.  Therefore, when an area with more in placed in an area with less moisture, moisture is transferred from the area of higher concentration to the area of lower concentration.  Over time, enough moisture is diffused from the timber into the air (or vice versa) so that the material neither gains nor loses moisture in the exchange. 

This is where the concept of ‘equilibrium moisture content’ (EMC) enters the scene.  At EMC, the moisture content of the timber is balanced, or in equilibrium, with the moisture content or humidity of the surrounding atmosphere. 

At a basic level, the wood fibres swell as they absorb moisture and shrink as they release it, causing the timber to expand and contract.  In the northern hemisphere, relative humidity increases in the summer and decreases in the winter. 

As a general guide, timbers will change 1% in size for every 4% change in surrounding moisture content levels.  

When a tree is first felled, the moisture content of the timber can be over 50%.  However, UK summer humidity is 15% so the timber will undergo a great change as it adjusts its moisture content down from 50% to 15%.   

Where Warping Happens

However, the change is not uniform across the timber.  There are many factors which cause uneven moisture changes.  Primarily, the outer areas of timber will dry out faster than the inside areas and the grain direction will also play a part.  These uneven movements cause one section to dry and faster than another, leading to the movements described earlier in this blog. 

The primary cause of visible warping of timber is the moisture content transition from fresh sawn to seasoned timber.  However, the diffusion process never stops – in the UK, summer humidity is 15% and winter humidity is 20% so the timber will continue to change on a seasonal basis although for external furniture, the seasonal changes will not be noticeable. 

Finally, the length of the timber will not change much even given extreme moisture content fluctuations – the changes are focused across the width and height of the grain.  This is because all timbers are made of tiny individual fibres which expand or contract across the width of the fibres rather than the length.  As seen in the diagram below, as the timber changes the moisture content from fresh sawn to EMC, the longitudinal direction, parallel to the grain, will only shrink 0.01% of its length.  However, across the grain the timber size will decrease between 4% and 8%. 

Factors affecting timber movement: 

Species 

Different timber species move in different ways.  Species known to be resistant to warping include tropical hardwoods such as Opepe and Iroko, Redwood and Cedar.  This is mainly due to a combination of relatively low fresh sawn moisture content and straight grain patterns.  Hardwoods tend to be more resistant to warping than softwoods.  A softwood known to be stable is Douglas Fir once the initial drop in moisture content has occurred and it has reached equilibrium with the atmospheric moisture content. 

Oak is a timber which is perceived to have a high risk of warping. If you want to find out more about Oak and how to mitigate the risk, read our blog here.

How the timber is cut 

The most common, and most cost-effective method of cutting timber is flat sawing.  This is where the entire log is cut horizontally lengthwise so the grain at the end of the board will have a horizontal pattern rather than a vertical pattern.   

The other methods of cutting timber are quarter-sawn and rift-sawn.  The yield vertical grain boards.  A flat sawn board will move around twice as much as a vertical grain board with the same change in moisture content.  This makes quarter-sawn and rift-sawn timber much more resistant to warping. 

However, because cutting quarter-sawn and rift-sawn timber is much less efficient, they are expensive and can be hard to find.  Most timber is flat sawn – if quarter sawn is desired, then it must be specified. 

Sapwood vs Heartwood 

In every trunk, there are two types of wood – heartwood and sapwood.  Heartwood is the innermost part of trunk which is technically ‘dead wood’ because no substances pass through it.  Sapwood is the smaller section of the trunk outside of the heartwood which transports the sap up and down the trunk.  Heartwood is always a darker colour than sapwood. 

This is very relevant to timber warping because sap wood actively carries the water whereas the heartwood does not.  Therefore, the fibres in the sapwood expand and contract much more quickly than the fibres in the heartwood making the sapwood more vulnerable to warping. 

Profile Size of the Timber 

As a rule of thumb, large sections of timber (end profile of larger than 150mm square) are safe from warping compared to small sections (end profile less than 80mm square).  This is due to the inherent strength of large sections of timber.  As warping is largely caused by uneven grain patterns, in large sections the grains will be going in several directions, so they essentially cancel each other out.  However, on smaller sections there will be a dominant grain angle which will exacerbate warping. 

Grain Patterns 

Timber is more stable when the grain is straight and close together.  This is partly why hardwoods are generally more stable than softwoods.  Hardwoods grow very slowly, over the course of hundreds of years.  This slow growth results in the growth rings being tightly packed making the timber more stable and less likely to move.  On the other hand, softwoods can reach maturity after 20-30 years which results in the growth rings being widely spaced. 

Wood Manufactured Materials 

Wood manufactured materials such as plywood and MDF will not noticeably warp or move.  They will do, but at a 1/10 of the rate of solid timber which is a negligible amount. 

Sources 

https://en.wikipedia.org/wiki/Wood_warping

https://www.delmhorst.com/blog/bid/364351/what-is-equilibrium-moisture-content-and-why-is-it-important

https://www.kaltimber.com/blog/2017/2/6/equilibrium-moisture-content-who-why-when-how

https://woodworking.stackexchange.com/questions/5871/which-direction-does-wood-expand

https://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/

https://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/2_Wood_Movement/2_Wood_Movement.htm

https://www.logic-sf.co.uk/blog/2018/02/timber-how-the-moisture-content-effects-it/

https://logic-bespoke.com/fresh-sawn-vs-seasoned-timber-stability-vs-availability/

https://homeguides.sfgate.com/stable-wood-against-warping-shrinking-99470.html

https://www.hunker.com/13401288/which-wood-warps-the-least

https://growitbuildit.com/how-to-buy-lumber-that-is-stable-and-doesnt-warp/

https://logic-bespoke.com/timber-durability-classes/

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